1. Field of the Invention
The present invention relates generally to flat-panel imagers for X-ray imaging, and more particularly to an active matrix flat panel imager.
2. Description of the Related Art
Active matrix flat-panel imagers (AMFPI) based on active matrix thin film transistor (TFT) arrays are the most promising technology for digital x-ray imaging due to their compact size, rapid readout, and better imaging performance than screen films. Active panel imaging based on flat-panel imagers are well known in the art. AMFPI are categorized as either direct or indirect based on the materials used for x-ray detection (x-ray photoconductors or scintillators, respectively).
FIG. 1 is a perspective view illustration of a conventional AMFPI with direct detection. The AMFPI employs a uniform layer of x-ray-sensitive photoconductor, e.g., amorphous selenium 101 (a-Se), to directly convert incident x-rays to a charge. Each pixel storage capacitor 109 stores each pixel charge. The charge is then electronically read out by a two-dimensional array of amorphous Silicon (a-Si) thin-film transistors 103 (TFT). During readout, a scanning control circuit 105 generates pulses to turn on TFTs one row at a time, and to transfer the image charge from the pixel to external charge-sensitive amplifiers 107. These amplifiers are shared by all the pixels in the same column. Advantages of the direct method include higher image resolution and simpler TFT array structure that can be manufactured in a standard facility for active matrix liquid crystal displays (AMLCD).
FIG. 2 is a perspective view illustration of a conventional AMFPI with indirect detection. A phosphor screen 201 is laminated upon a two-dimensional array such that one planar surface of the phosphor can be radiated by incident x-rays and the opposite planar surface of the phosphor can transfer photons which are detected by the adjacent photodiode array 203. Suitable phosphor screens include structured cesium iodide (CsI). In operation, the phosphor screen converts incident x-ray radiation to optical photons, which are detected by the photodiode array and are then converted to charge by integrated photodiodes at each pixel of the TFT array 205. A scanning control circuit 207 then generates pulses to turn on the photodiodes one row at a time, and to transfer the image charge from the pixel to external charge-sensitive amplifiers 209. These amplifiers are shared by all the pixels in the same column.
Both AMFPI methods offer better image quality than screen films and computed radiography (CR).
Existing flat-panel imagers (FPI), which are the dominant technology for digital x-ray imaging, have two major difficulties to overcome: the ability to generate good image quality at very low dose, such as in the dark part of a fluoroscopy image (˜0.1 mR per frame) or behind dense breast tissue in mammography, and the ability to produce images at a high frame rate without artifact, especially when the radiation exposure is switched from radiographic to fluoroscopic. This is because of the “ghost” generated by the previous exposures.
Several strategies exist for improving the low-dose performance of FPI. These can be divided into two categories: increasing the x-ray image charge conversion gain so that the signal can overcome the electronic noise, and decreasing the electronic noise. Theses strategies are common to both types of FPI since they have approximately the same conversion gain and pixel electronic noise. Comparing the two approaches, increasing the gain has the potential for generating more significant improvement.
One known method of increasing the gain for direct FPI is to use photoconductors with higher conversion gain, e.g., lead iodide (PbI2) or mercuric iodide (HgI2), which have conversion gains 5-7 times higher than that of amorphous Selenium (a-Se). One of the practical problems of having large gain is that the signal charge, especially at high exposure, requires a large pixel storage capacitor (>15 pF), which is impractical to produce especially for small pixel sizes. This means that a detector for low dose x-ray imaging applications may not work properly with a high radiation dose, which compromises the dynamic range of the system and its clinical applications.
To reduce electronic noise, several investigators have proposed advanced pixel designs, which incorporate pixel amplification by adding at least two more TFTs at each pixel. This has been found to be impractical to implement the complex pixel design over a large area with consistent and uniform imaging performance because each pixel operates as an analog amplifier circuit as opposed to the simple switching device (digital) in existing AMFPI.